1. Field of the Invention
The present invention relates to a torque detection apparatus such as a torque detection apparatus used in a electric power steering (EPS) developed for reducing the vehicle steering effort.
2. Description of the Related Art
Conventionally, a magnetostrictive torque sensor has been used for detecting a steering torque required for operation control of the electric power steering (EPS).
As disclosed in Japanese Patent Publication 8-91236 and Japanese Patent Publication 2-195221 [2], this type of torque sensor is provided with a signal processing circuit for processing a predetermined electric signal from the torque sensor and outputting a torque detection signal.
Document [1] discloses a new technique associated with a steering feeling of an electric power steering (EPS). More specifically, a torque sensor circuit signal and a vehicle speed signal are input to the signal processing circuit and according to the vehicle speed, a phase compensation (delay or advance compensation) is performed by software on the torque sensor circuit signal, so that the steering feeling is improved and the steering hunting is prevented.
In this case, the software phase compensation, specifically, has a configuration so as to execute the software phase compensation by switching between predetermined constants according to the vehicle speed.
Moreover, document [2] sets as an important target to stabilize a signal output from a torque sensor. Especially when the signal processing circuit has failed and the output of the signal processing circuit is abnormally increased or lowered, the output signal of the signal processing circuit is forced to be grounded, so as to forcibly interrupt output of an incorrect torque detection signal. That is, the operation control of the electric power steering (EPS) is substantially interrupted, so that the steering hunting is prevented.
However, the torque sensor disclosed in document [1] has a problem as follows. For example, when a hunting has occurred due to a failure of a component of the magneto-striction torque sensor or a part of the EPS controller, it is impossible to cope with the hunting only by switching torque sensor circuit signal from a predetermined constant to another according to the vehicle speed so as to perform a software phase compensation.
Moreover, in the torque sensor disclosed in Document [2], as has been described above, when an error is detected, the torque detection signal is immediately made zero to interrupt the steering operation control by the EPS. Accordingly, if an external noise is mixed in the torque sensor signal to instantaneously increase or decrease the signal processing circuit output abnormally, there arises a problem that the signal processing circuit output signal is immediately forced to be grounded to be come 0, which interrupts the control operation by the EPS controller.
It is therefore an object of the present invention to provide a torque detection apparatus that is capable of catching an error signal output from a signal processing circuit with a high accuracy, and properly performing an interrupt control of the control operation by an EPS controller, so that the EPS can have an effective fail-safe characteristic.
The torque detection apparatus has a torque sensor arranged on a rotary shaft that outputs a predetermined electric signal according to a torque applied to the rotary shaft. A sensor circuit outputs the predetermined electric signal output from the torque sensor, as a sensor circuit signal. A signal processing circuit processes an output from the sensor circuit and outputs the processed output as a torque detection signal. An abnormal state monitoring circuit arranged adjacent to the signal processing circuit operates according to the sensor circuit signal so as to monitor a presence/absence of an abnormal operation.
The abnormal state monitoring circuit includes a signal monitoring circuit that monitors a presence/absence of a hunting state in the sensor circuit signal, a control circuit that operates upon detection of the hunting state to forcibly converge an output value of the torque detection signal to a predetermined value, and an error signal output circuit which outputs a predetermined level error signal, such as for example, a High level error signal or a Low level error signal, to the EPS controller.
The aforementioned configuration is arranged, for example, adjacent to an EPS controller of an electric power steering and outputs the torque detection signal to the EPS controller. According to this torque detection signal, the EPS controller urges an auxiliary steering torque to the power steering apparatus.
The torque detection signal is generated in a signal processing circuit by a signal processing of a sensor circuit signal based on the torque sensor output. Moreover, simultaneously with this, the sensor circuit signal is used to monitor the presence/absence of the hunting state in the signal monitoring circuit of the abnormal state monitoring circuit.
The hunting state of the sensor circuit signal is caused by a failure of the torque sensor (or the sensor circuit, or a control error of the EPS controller). By monitoring this hunting state, an error generated in the respective components can be detected.
When the hunting state is generated in the sensor circuit signal, the torque detection signal output form the signal processing circuit is also in a hunting state. The control circuit of the abnormal state monitoring circuit operates to gradually reduce the hunting amplitude of the hunting torque detection signal. Accordingly, a torque control by the EPS is also converged in its urge amount.
After this, an error signal is output by the error signal output circuit to the EPS controller. For example, upon reception of this error signal, the EPS controller stops a torque urge control of the power steering apparatus.
According to a feature of the invention, the signal monitoring circuit includes a hunting state identification function which determines whether a signal associated with the sensor circuit signal is in a hunting state. An output suppressing function operates when the signal associated with the sensor circuit signal is determined to be in the hunting state and controls, via the control circuit, the output level of the signal processing circuit, so that the output level is at a predetermined middle point potential. An error signal output control function operates when a predetermined period of time has elapsed after the output level of the signal processing circuit is controlled to be at the middle point potential, and forces the error signal output circuits to output a High level or a Low level error signal.
According to another feature of the invention, when the hunting state is generated, the output level of the torque detection signal is adjusted to be the middle point potential. Accordingly, in the EPS controller, the drive torque urging the power steering apparatus is gradually converged to 0. After this, an error detection signal is output, and upon reception of this signal, the EPS controller terminates the torque urge control of the power steering apparatus.
According to an advantage of the invention, the signal monitoring circuit is supplied with the sensor circuit signal output from the sensor circuit, via a hunting information detection circuit that includes a high pass filter.
According to another advantage of the invention, the high pass filter operates in such a manner that only the sensor circuit signals having a frequency band higher than a predetermined value are output to the signal monitoring circuit. The signal monitoring circuit monitors the presence/absence of the hunting state associated with the sensor circuit signals of only this frequency band.
Here, the pass frequency of the high pass filter is preferably set to a frequency band where the hunting is easily caused.
The hunting state identification function of the signal monitoring circuit operates and decides that the output of the sensor circuit is in a hunting state when a fluctuation frequency associated with the sensor circuit signal output from the sensor circuit is gradually lowered and the amplitude of the output signal is gradually increased with a fluctuation exceeding a predetermined potential.
In addition, the signal monitoring circuit converges the output of the sensor circuit signal to the middle point potential and outputs an error signal only when the aforementioned three phenomena are detected, i.e., the lowering of the fluctuation frequency, the increasing of the amplitude, and a fluctuation exceeding a predetermined potential.
The signal monitoring circuit includes an amplitude fluctuation measurement function that determines whether the amplitude of the sensor circuit signal exceeds a reference value for a period of time exceeding a predetermined value, and operates to execute the hunting state identification function of the signal monitoring circuit when the amplitude fluctuation measurement function has determined that the amplitude of the torque detection signal has exceeded a predetermined reference value for a predetermined period of time.
In this invention, the amplitude fluctuation measurement function is used to determine whether the amplitude of the sensor circuit signal is greater than the reference value. If the determination is positive (yes), the state continuation time is counted. When this continuation time exceeds a predetermined period of time, it is determined whether a hunting state is present.